The separation of water from either hydrocarbon or halogenated hydrocarbons contaminants found in chemical industrial waste or from gaseous hydrocarbons generally requires a separation system capable of prolonged exposure to hydrocarbons without chemical degeneration or fouling. Chemical degradation is the attack of the hydrocarbon or halogenated hydrocarbon on the chemical components of the separation device, such as the separation membranes, generally leading to chemical degradation and failure of the membrane system. Michaels writing in "Ultralfiltration Membranes and Applications", "Polymer Science and Technology", vol. 13, 1979, describes fouling as a phenomenon whereby a membrane, under normal operating conditions develops a resistance to flow and severely limits transmembrane permeate throughput (concentration polarization).
Another general aspect involved in water-hydrocarbon separations are the problems that arise from the separation of molecules that are approximately the same size. For example, water molecules are about the same size as methane molecules. Hence, porous membrane systems having the most microscopic porosity presently available are not capable of distinguishing between water and methane. Other membranes or otherwise chemically treated membranes which are hydrophilic still pass water and methane through their pores because of the flow dynamics of the water through the pores of the membrane.
Water vapor is often found as an impurity in industrial gas streams, requiring its removal before or during use or processing of the gas streams. For example, natural gas, the principal component of which is methane, contains in its natural state substantial quantities of physically entrained water. In numerous industrial processes it is desirable or necessary that such gas be dried.
Conventional prior art processes for drying natural gas have exploited the solubility of water in certain substances, such as methanol or glycol, by passing the wet gas through a bath or curtain of the scrubbing fluid. In such processes, the scrubbing fluid, when saturated, is regenerated by driving off the water, usually by heat generated by the combustion of well-head gas. The costs inherent in such processes include the costs of the well-head gas used in regeneration of water-saturated sorbent liquid as well as the cost of sorbent liquid itself.
In other prior art arrangements, molecular sieve adsorbents are used, the wet gas typically being passed through a bed providing the molecular sieve. Here again, the need for regeneration or replacement of the bed gives rise to substantial energy costs.
Desiccation using solid adsorbents, and mechanical operations (cooling or refrigeration to induce precipitation) have also been used.
The U.S. Pat. No. 3,735,559 to Salemme, issued May 29, 1973 discloses sulphonated polyxylyene oxide membranes for the separation of water vapor from other gases. The membranes are supported by a support structure which obstructs portions of the membrane surface. The flat membranes are supported between two base screens forming a sandwiched flat membrane assembly. The application further discusses problems encountered with such membranes, such as ruptures and linear shrinkage.
The U.S. Pat. No. 4,421,529 to Revak et al, issued Dec. 20, 1983, discloses a process for separating gases using hollow fiber membranes on an intermittent basis. The patent discloses that the hollow fibers can be made from various materials, including cellulose esters or ethers, and preferably asymmetric cellulose acetate. These membranes are porous membranes which literally filter the hydrocarbon fluid. By being porous, hydrocarbon is able to flow through the pores. Additionally, these membranes are approximately 250 microns thick, the thickness of the membrane contributing negatively to the flow dynamics of the system.
As disclosed in the article entitled "Reverse Osmossis: A New Field of Applied Chemistry and Chemical Engineering", by S. Sourirajan, the founder of cellulose acetate membranes, published in "Synthetic Membranes", vol. 1, 1981, cellulose acetate membranes comprise an asymmetric porous film. Such a porous film is used for preferential absorption of one of the constituents of a solution at the interface of the film. It is undisputed, as shown by electron microscopy, as shown by Kesting in FIG. 2.17 page 40 of "Synthetic Polymeric Membranes", 1971, cellulose acetate ultrafiltration membranes are a cohesive system consisting of open celled foams, i.e. vacules with breached walls. Fastening the cellular network together are long ribs extending in three dimensions.
Cellulose acetate and cellulose ether membranes dissolve or disintegrate in the presence halogenated hydrocarbons. The hydrocarbons tend to chemically degrade the exposed ether or acetate groups exposed to the hydrocarbon from the surface of the membrane.
The U.S. Pat. No. 3,442,002 to Geary, Jr. et al, issued May 6, 1969, discloses a method of manufacture of fluid separation apparatus wherein the apparatus may include a plurality of separation modules.
The U.S. Pat. No. 2,981,680 to Binning, issued Apr. 25, 1961, discloses a process for separating molecular solutions of a mixture of components, the process including the utilization of two or more dissimilar (having different compositions) permeation membranes. The patent discloses the use of "regenerated" cellulose (type 300PT). Type 300PT cellulose is a cellulose acetate. The terminology "regenerated cellulose" is used in the patent to mean chemically modified reconstituted cellulose. The term "regenerated cellulose" is usually meant to more strictly mean unchemically modified cellulose such as that produced by the viscous or cuproammonium regeneration processes. Cellulose regenerated by the latter two mentioned processes is substantially similar to natural cellulose, the regenerated cellulose from these processes not being chemically altered. Chemically altered cellulose membranes, such as cellulose acetate, are used for producing porous membranes, usually asymmetric membranes for filtration processes. These chemically altered membranes are susceptible to degradation in the presence of hydrocarbons, such as methane.
The U.S. Pat. No. 3,735,558 to Skarstrom et al discloses a process for separating fluids and an apparatus to be used therewith. The apparatus separates water vapor from air by creating a pressure gradient across the walls of permeable tubes to induce permeation therethrough. A counter current reflux flow induces a longitudinal concentration gradient along the walls of permeable tubes which enhances permeation of key components through the walls of the tubes thereby separating them from a mixed fluid feed. The Skarstrom et al patent does not disclose the use of cuproammonium cellulose membranes nor does it address the problems of separating hydrocarbons from water. Finally, the Skarstrom et al patent discloses the use of semipermeable membranous hollow members which are porous as opposed to a nonporous separating membrane.
The Japanese patents 13,653 issued Feb. 1, 1979 and 152,679 issued Dec. 1, 1979 both disclose the use of cuproammonium rayon to selectively pass water vapor therethrough. Although the Japanese references, just as the other references discussed above, disclose the ability of cuproammonium cellulose membranes to pass water therethrough, there is no disclosure or suggestion of the ability of cuproammonium cellulose membranes to withstand continued and prolonged exposure to a hydrocarbon or halogenated hydrocarbon water mixture without fouling or chemical degeneration of the membrane.
The present invention relates to a diffusion type membrane which is capable of being made into ultra-thin fibers thereby contributing positively to the flow dynamics by presenting a minimal path for the water separated from the hydrocarbon to travel. The present invention further provides an unsupported membrane requiring no additional support which blocks portions of the membrane from direct contact with the flowing hydrocarbon fluid. The invention further provides a membrane that is unexpectedly resistant to degradation in the presence of hydrocarbons, such as methane. Thus, the present invention provides an apparatus and method for significantly more efficiently separating water and dissolved water soluble components from a hydrocarbon fluid.